In the manufacture of a printed circuit board assembly, a plurality of electrical components are mounted on a printed circuit board by extending leads of the components through apertures in the board from one side of the board. Projecting portions of selected ones of the leads on an opposite side of the board then are crimped over to retain the components on the board, with the remaining leads projecting from the opposite side of the board. The leads then are soldered to contact pads surrounding the apertures, and protective solder coatings are formed on conductor paths of the board, by passing the board assembly over a molten solder wave, such as the dual solder wave disclosed in the U.S. Pat. No. 4,101,066, issued July 18, 1978, to V. A. Corsaro and E. A. Gutbier.
As is known in the art, a molten solder surface tends to oxidize even when traveling in a molten solder wave, and thus has an inherent high surface tension. As a result, as the printed circuit board assemblies pass over the solder wave, unless a suitable fluxing agent is employed, the molten solder in the solder wave tends not to separate between adjacent contact pads and/or conductor paths, thus producing solder defects in the form of solder crossovers or bridges (i.e., shorts) between the contact pads and/or conductor paths. This is particularly true where adjacent contact pads and/or conductor paths are close together, such as on the order of one-tenth of an inch or less. This producing of solder defects is further aggravated where only selected leads of the components are crimped as noted hereinabove, since the remaining projecting leads tend to produce "drag out" of solder as the leads exit from the solder wave. Accordingly, it is standard practice to apply a suitable soldering flux to the printed circuit board assemblies prior to their passage over the molten solder wave, and/or to spray a soldering flux into the point at which the printed circuit board assemblies exit from the wave, which is known in the art as a "peel back" region. The degree to which a particular soldering flux then reduces solder crosses in the soldering operation is dependent upon the activation level of the flux, that is, the ability of the flux to reduce oxidation and surface tension formation in the solder wave in the "peel back" region.
In the past, the activation level of different fluxes and their soldering performance, and thus the capability of a solder wave to produce properly soldered printed circuit board assemblies when one of the fluxes is used in a soldering operation, has been determined by conducting extensive soldering quality yield studies. This is disadvantageous because of the time and expense involved, and because the studies do not provide for periodic "on site" indications of the capability of a particular solder wave, with which a selected flux is being used, to produce properly soldered printed circuit board assemblies under variable process conditions.
Another known procedure for determining the capability of a solder wave to produce properly soldered printed circuit board assemblies involves the engaging of a transparent gauge plate with the solder wave in the same manner that the printed circuit board assemblies engage the solder wave in a soldering operation, and then observing the physical behavior characteristics of the solder wave as the wave impinges on the transparent plate. This procedure, however, produces unreliable results because the behavior of the molten solder wave as the wave impinges on the solder-nonwettable surface of the transparent gauge plate does not correspond to the solder wave impinging on solder-wettable metal surfaces, such as the conductor paths of a printed circuit board assembly.
Accordingly, a purpose of this invention is to provide a new and improved procedure for making an "on-site" determination of the capability of a solder wave to solder printed circuit board assemblies with a minimum number of solder defects, by emulating more closely the physical behavior characteristics of the molten solder wave in an actual soldering operation.